1. Field of the Invention
This invention relates generally to photovoltaic solar cells and to a method of making the cells. More particularly, the invention relates to the use of certain transparent films as anti-reflection or index-matching layers in solar cells.
2. The Prior Art
Solar cells in commercial use today are of generally two distinct types--cells based on wafers of silicon crystals and cells based on thin-film technologies, in particular, cadmium sulfide solar cells. In all solar cells, one of the major goals is to produce a cell that has a long life and does not degrade upon exposure to the environment. For example, condensed acids from rain, humidity-induced galvanic corrosion and other noxious atmospheric contaminants can cause corrosion of the cell. Accordingly, most cells are encapsulated in glass-covered, hermetically sealed cases. Examples of a variety of such cases are disclosed in U.S. Pat. Nos. 3,912,540; 3,989,541 and 4,036,645.
One problem that occurs when a smooth glass plate is used as the cover plate in a solar cell is that a non-uniform contact between the irregular cell surface and the glass results. Wherever there is a lack of true optical contact, air is included in the space. The difference in index of refraction of these volumes (air and glass) is such that some 8 to 10 percent of the incoming solar energy may be reflected away from the solar collector surface. Thus, it is desirable to fill the gaps between the glass and the cell with a material having an index of refraction greater than or equal to the glass cover plate in order to reduce reflection. Such materials, termed anti-reflection layers, have been employed in covered cells in the past. However, many of these layers have certain shortcomings, such as lack of stability and lack of easy workability. What is needed is some type of filler or anti-reflection layer that is transparent, stable under exposure to light, forms a good bond to both the glass and cell surface material, and is easy to work with.